The invention relates to a separating drum for the centrifuging of liquid mixtures including a specifically heavier liquid and a specifically lighter liquid, and containing solids, in which after a substantial separation of the specifically lighter liquid phase, the specifically heavier liquid containing some of the solids is subjected to a recentrifuging in a second separating chamber.
The centrifuging of liquid mixtures is performed mainly in known separating drums in which the separating chamber comprises a plurality of conical disks having narrow interstices between them, which is known as a disk stack, the zone of separation between the specifically lighter and specifically heavier liquid being established further inwardly or outwardly in the disk stack according to the proportions of the two phases and the desired degree of separation. If the proportion of the lighter phase is smaller, the zone of separation is shifted inwardly, and if it is greater, the zone of separation is shifted outwardly. Normally, the proportion of the specifically heavier phase is greater than that of the specifically lighter phase when an inwardly located zone of separation is established, and is smaller when an outwardly located zone of separation is established. The location of the zone of separation is established when the drum is designed, and depends on the disposition of the ascending passages in the disk stack, and also depends on the application of the centrifuge.
While the separated liquids are removed from the drum in any desired manner, e.g., by means of paring disks, the solids that are present are spun out in the centrifuging chamber, provision often being made for a preliminary separation of the solids in the distributor chamber or in a preliminary separating chamber.
German Auslegeschrift No. 1,075,047, for example, shows an embodiment of this kind.
Depending on the proportion of the solids, a drum having an imperforate peripheral wall or a self-cleaning centrifuge drum can be used, and while a good separation of all three phases is desired, that is accomplished only in the rarest cases.
Let us consider as an example the separation of oil and water mixtures, such as is commonly performed to a great extent on ships. These liquid mixtures contain the light phase, i.e. oil, in major proportion and heavy phase, e.g. water, in minor proportion. These liquid mixtures must furthermore be centrifuged at elevated temperatures in order to achieve good separations.
In this known separating process, more value has hitherto been placed on a precisely separated and water-free oil phase than on an oil-free water phase. Today, however, not only the oil losses resulting from this practice but also the protection of the environment are important, so that the water phase, too, is to be as free as possible from oil.
This requirement, however, is difficult to meet with the centrifuges commonly used, because in order to achieve the essentially required water-free oil, the zone of separation between the two liquids must be situated very closely to the outer margin of the disk stack so as to provide, on the one hand, a large separating surface area in the disk stack for the oil phase, i.e., for the lighter liquid phase, and on the other hand to prevent the solid substance from having to traverse a longer path within the disk stack and thus foul or clog the interstices in the disk stack. A clogged disk stack shortens the run time of the centrifuge, or else more frequent desludging operations have to be performed to clean out the drum, and this in turn leads to losses of oil. This procedure, namely the shifting of the zone of separation to the outer margin of the disk stack, results, of course, in a more poorly separated heavier phase. In addition, the solids, which often settle only in the distribution chamber or in an antechamber, have particles of oil attached to them, which are not separated until they reach the outer centrifuging chamber, and then are entrained by the outflowing heavier phase. Poor separations occur especially when the mixture is emulsion-like, when the necessary temperatures are not maintained, or when the centrifuge is momentarily overloaded. In many cases, therefore, an additional centrifuge is used for secondary clarification.
British Pat. No. 565,713, shows the use of centrifuge drums having a plurality of superimposed separating chambers, a preliminary clarification or separation of the liquids being performed in the one chamber, and a secondary centrifuging being performed in the next chamber.
The above designs in each case reduce the separating surface of the centrifuge drum for one phase, e.g., the lighter phase, so that the throughput of the centrifuge with respect to the total volume of the drum is greatly reduced, and thus the separating process is made substantially more costly.